Extrinsic hydrophobicity-controlled silver nanoparticles as efficient and stable catalysts for CO2 electrolysis

To realize economically feasible electrochemical CO2 conversion, achieving a high partial current density for value-added products is particularly vital. However, acceleration of the hydrogen evolution reaction due to cathode flooding in a high-current-density region makes this challenging. Herein, we find that partially ligand-derived Ag nanoparticles (Ag-NPs) could prevent electrolyte flooding while maintaining catalytic activity for CO2 electroreduction. This results in a high Faradaic efficiency for CO (>90%) and high partial current density (298.39 mA cm‒2), even under harsh stability test conditions (3.4 V). The suppressed splitting/detachment of Ag particles, due to the lipid ligand, enhance the uniform hydrophobicity retention of the Ag-NP electrode at high cathodic overpotentials and prevent flooding and current fluctuations. The mass transfer of gaseous CO2 is maintained in the catalytic region of several hundred nanometers, with the smooth formation of a triple phase boundary, which facilitate the occurrence of CO2RR instead of HER. We analyze catalyst degradation and cathode flooding during CO2 electrolysis through identical-location transmission electron microscopy and operando synchrotron-based X-ray computed tomography. This study develops an efficient strategy for designing active and durable electrocatalysts for CO2 electrolysis.

The present work of Hyung-Suk Oh and co-workers reports the CO2 electrolysis performance of Ag nanoparticles obtained by a colloidal synthesis followed by partial ligand removal.The importance was given to the effect of the synthesis-derived-ligands on the long term stability for CO2 in effort to minimize electrowetting at high current density testing.Most of the figures are clear and there are several operando methods (Synchrotron X-ray CT and XANES) to integrate visual aid for the catalytic activity and stability which are novel to the field of CO2 electrolysis.Yet, the info provided in the methods and SI are not detailed enough and needs further inputs.There are some logical leaps in the interpretation of the data, especially amongst the samples.At the current state, the conclusions do not seem to present a major advancement in the current state-of-art.Thus, the work is recommended to be go under significant revision after considering the following questions and comments; Questions 1.The sentence in page 3 claims that a. "… electrocatalysts with hydrophobic properties have not yet been reported…".
Several groups have demonstrated the influence of long-chain ligands derived from the colloidal nanoparticle synthesis of Cu, Ag and CuAg such as oleylamine1 and nitro-containing ligands2, displaying a hydrophobic property in CO2 electrolysis by capacitance and zeta-potential measurements, respectively.Therefore the authors need to rephrase this sentence by assessing the results in the established literature.
b. "… Furthermore, the lipid ligand inhibits Ostwald ripening and sintering of the electrocatalyst during the CO2RR, thereby facilitating the maintenance of the fine nanoparticle size…" This phenomenon (catalyst sintering) can be explained by the collapse of the ligand upon its introduction into an aqueous solution.In particular, CO2 electrolysis is known to be highly alkaline at the local reaction environment, pH > 11 at high current densities > 200 mA/cm2 3.In such conditions, the ligands may undergo hydrolysis and the conformal arrangement of Ag-NPs would collapse.This has been demonstrated by ex-situ ligand exchange studies in this article: ACS Catal.2020, 10, 22, 13468-13478 1 2. Collodial synthesis in OLA-type solutions require hazardous chemicals during the synthesis and the purification steps.On top of that, the yield of the synthesis are usually 30 % and below in terms of grams of metal used.After 50 hours, the selectivity and partial CO current density are not drastically different between the Ag-NPs and commercial Ag-PTFE samples.Could the authors provide further disucssion on the novelty in that aspect? 3. Refering the line in Page 5: " … To analyze the hydrophobic properties while maintaining the morphology of the catalyst, by leaving a part of the ligand, physical property analysis was performed using Ag-NP treated with TMAH for 2 h.…" a. Figure S2 shows the HR-TEM images of the "OLA-capped Ag-NPs" after 5 h of TMAH treatment to remove OLA, which results with the collapse of the conformal arrangement and the size of Ag-NPs.Hence, an alternative of 2 h TMAH treatment was suggested to preserve the NP stability.However, the evidence for the stability after 2h TMAH treatment is limited -so far, this evidence is limited to three individual Ag particles in Figure S3.Could the authors please provide further evidence with HR-TEM images of the selected process (2h-TMAH), comperable to the images in Figure S2 (or in Fig. 1a)? which are the TEM images used to build the histograms of 2h-TMAH sample? 4. Eventhough Atom Probe Tomography (APT) technique seems to be a strong indicator of the presence of N-containing ligands (such as oleylamine-OLA and tetramethylammonium-TMAH), a conclusive analysis for the presence of the surface groups would be the Fourier Transform Infrared Spectroscopy (FTIR) or the analysis of the colloidal solution by Nuclear Magnetic Resonance (NMR).The N-signal observed by APT could be derived from OLA and/or TMAH, therefore could the authors reach a conclusion at this end and provide stronger evidence about the content of the surface chemistry?
5. There is a logical leap of using ligands, and the going to partial ligand-exchange/removal and then an oxidation step… What is the motivation for the authors to form an oxide-derived Ag-NPs, instead of the as-synthesized Ag-NPs? 6. FTIR spectra of the OD-Ag NPs were given Figure S4.The preparation method of this sample for the FTIR analysis is not clear, please specify in the experimental section.The sustanion chemistry may also give signals around 2600 -3200 cm-1 wavenumbers4.After the disassembling the MEA, could the authors be certain that those peaks are only belong to the synthesis ligands of the Ag-NPs?Is there a control sample where commercial Ag-NPs were analyzed by FTIR after an oxidation step in a similar MEA assembly?7. Regarding Figure S8, a.The Faradaic efficiencies and partial current densities do not match.For ex. at 3.6 V, Ag black shows 70 % FE of H2 and 500 mA/cm2 total cell current, which should be equal to 350 mA/cm2 partial current for H2.Yet, Figure s8c shows only 275 mA/cm2.Could you please explain the inconsistency in the efficiency data?Please present a table of results for the tests conducted.b.There are not any error bars in any of the data.Is this a single test run?Is there any deviation amongst the GC injections -whether there are multiple injections or not?And are there any duplicate or triplicate tests to minimize the deviation?8.In page 7, please clarify or rephrase this statement, as it is inconclusive with the prior sentence."…Therefore, the partial current density of CO exhibited by the Ag-NP catalyst at low cathodic overpotentials was significantly smaller than the values at high cathodic overpotentials…" 9.The size of the Ag-black particles need to be clarified, as the authors claim a higher activity for Ag-NPs due to its "moderate size between 5 -10 nm".How is this an indication of "…extrinsic effect of the lipid ligand..", please clarify?10.In Figure S9, a.The partial current densities do not add up to the total value.For ex. at 3.4 V, the total j is around 300 mA/cm2, while the jCO is between 220 -250 mA/cm2 and jH2 is almost zero, please clarify the breakdown of the calculation, preferably with table of results?11.Regarding line " .. This is due to a decrease in the active surface area at the expense of suppressing the flooding phenomenon at a high cell voltage…" a. Are there any ECSA measurement to support this claim?b.For Ag-PTFE, the total cell current is less compared to Ag-NPs, which points to a higher resistance udner fixed voltage testing.Hence, the authors need to consider the electrical resistance contribution of the PTFE additive instead.12.In Figure 2 b.Please provide information about the cell configuration and electrodes used for the ex-situ analysis here.
14.In page 10, regarding line "… They expose the high-crystallinity (111) plane, inducing excellent catalytic activity toward the CO2RR under cation-assisted conditions32,33, enhancing the CO2RR activity without cathodic corrosion…."This postulation needs further evidence such as; the authors need to justify this postulation with a before and after sample analysis with XRD pattern.The Ag (110) facet was shown by both experimental and theoretical work to be more active than the very stable (111) facet5.
15.The part about the XANES spectra in page 10, 2nd paragraph needs further discussion.It is advised to provide a brief information about the Operando X-ray absorption spectroscopy for the broader audience, as in Ref 5.The parameters such as the sample tilt needs to be addressed as it effects the penetration depth of the X-rays from 2 nm to 12 um at 0 to 45 angles.
a.The analysis of the results are not easy to follow such as the meaning of LCF is not explained.For ex. the quantification methods and the data analysis details needs to be reported in the SI and/ or the fitting results of the reference metal, metal-oxides and samples of Fig  Main manuscript Page 16, Line 30: Considering the diffusion time of generated gases in the device to GC, the measurements was started 9 min after the reaction start for each voltage.The measurement time is 13.5 min.
Comment 6: Mention the duration or the applied charge density of the electrolysis experiments performed to obtain the IL-TEM images (Fig. 3a-d).Furthermore, there is no experimental description of the IL-TEM experiments.What kind of cell was used to carry out those experiments?
Response: We are thankful to the reviewer for this valuable comment.Detailed experimental methods for IL-TEM analysis, including duration of electrolysis and reaction system, were added to the corresponding section in the revised manuscript.

Main manuscript Page 17, Line 26: IL-TEM analysis.
All electrochemical experiments for IL-TEM analysis were performed in a conventional three-electrode system with homemade PEEK cell.Ag/AgCl (3M NaCl) and a graphite rod were used as reference and counter electrodes, respectively.Ag black and Ag-NPs inks were drop-casted on a holey carbon-coated Au grid (Agar scientific, H7 finder grids) and put it on rotating disc electrode (RDE; VSP, Bio-Logic Science Inc.) in a customized holder and the grid was fixed by screwing a PEEK cap to secure electrical contact.The electrochemical measurements were carried out in CO2-saturated 0.1M KHCO3, the potential was converted to the RHE scale as following equation: TEM images were collected at the same location of Au grid, and the morphology of the catalysts was compared before and after CO2RR relying on the applied potentials (-1.0 VRHE) and the reaction time (4 h) while at a rotation speed of 1600 rpm.
Comment 7: The colors of the Metallic-Ag, Ag + , and Ag 2+ mentioned in the caption of Fig. 3g (LFC) do not correspond to the colors observed in the plot.
Response: We are thankful to the reviewer for this valuable comment.Indeed, it was a typo; we corrected caption of Fig. 3g in the revised manuscript.Comment 8: What was the electrolysis time or applied charge (at the different applied potentials) of the operando synchrotron computed tomographs observed in Fig. 4b?
Response: We are thankful to the reviewer for this valuable comment.The electrolysis time for the insitu/operando CT analysis was 10 min.Related phrase was added to the CT experiment section in the revised manuscript.

Main manuscript Page 19, Line 7:
For in-situ/operando CT analysis, each potential was applied for 10 min.
Comment 9: On page 12, it is mentioned that "The segmented electrolyte within the electrode surface is shown in Fig. 4d", I think it is 4b instead of 4d.

Response:
We are thankful to the reviewer for this valuable comment.Indeed, it was a typo; we corrected typo in the revised manuscript.

Main manuscript Page 13, Line 12:
The segmented electrolyte within the electrode surface is shown in Fig. 4b.

Reviewer: 2
Recommendation: The present work of Hyung-Suk Oh and co-workers reports the CO2 electrolysis performance of Ag nanoparticles obtained by a colloidal synthesis followed by partial ligand removal.
The importance was given to the effect of the synthesis-derived-ligands on the long term stability for CO2 in effort to minimize electrowetting at high current density testing.Most of the figures are clear and there are several operando methods (Synchrotron X-ray CT and XANES) to integrate visual aid for the catalytic activity and stability which are novel to the field of CO2 electrolysis.Yet, the info provided in the methods and SI are not detailed enough and needs further inputs.There are some logical leaps in the interpretation of the data, especially amongst the samples.At the current state, the conclusions do not seem to present a major advancement in the current state-of-art.Thus, the work is recommended to be go under significant revision after considering the following questions and comments; Comment 1: The sentence in page 3 claims that a. "… electrocatalysts with hydrophobic properties have not yet been reported…".
Several groups have demonstrated the influence of long-chain ligands derived from the colloidal nanoparticle synthesis of Cu, Ag and CuAg such as oleylamine 1 and nitro-containing ligands 2 , displaying a hydrophobic property in CO2 electrolysis by capacitance and zeta-potential measurements, respectively.Therefore the authors need to rephrase this sentence by assessing the results in the established literature.
b. "… Furthermore, the lipid ligand inhibits Ostwald ripening and sintering of the electrocatalyst during the CO2RR, thereby facilitating the maintenance of the fine nanoparticle size…" This phenomenon (catalyst sintering) can be explained by the collapse of the ligand upon its introduction into an aqueous solution.In particular, CO2 electrolysis is known to be highly alkaline at the local reaction environment, pH > 11 at high current densities > 200 mA/cm 2 . 3In such conditions, the ligands may undergo hydrolysis and the conformal arrangement of Ag-NPs would collapse.This has been demonstrated by ex-situ ligand exchange studies in this article: ACS Catal.2020, 10, 22, 13468-13478

Response:
We are thankful to the reviewer for this valuable comment.a.We modified the phrase based on the references provided by the reviewer.
Main manuscript Page 3, Line 13: However, even with the abovementioned major issues, research in this field has been limited to the microstructural control of electrodes or the introduction of polytetrafluoroethylene (PTFE) 17,18 ; few electrocatalysts with hydrophobic properties have been reported.Several studies introduced the influence of hydrophobic ligand in CO2RR. 19,20However, the real-time observation for the relationship between the behavior of electrolyte and CO2RR property according to hydrophobicity at three-phase boundary in the device have not yet been reported.
b. OLA is removed by treatment with a high concentration of TMAH solution for several hours.Therefore, it is hardly removed at pH 11~12, which is the local pH formed in the CO2RR.This can be indirectly comfirmed through XRD before and after the reaction.There was no change in particle size in XRD before and after the CO2RR.Also, there was no change in the crystalline structure.However, in the case of CO2RR using alkaline electrolyte with high-concentration, OLA may be slowly removed.Related contents added to the revised manuscript.
R8 Supplementary Figure 23.Wide-angle XRD patterns before and after CO2RR for Ag-NP catalysts.
Main manuscript Page 10, Line 19: As shown in Supplementary Fig. 23, there was no change in crystallinity or particle size for Ag (111) after CO2RR.In addition, the Ag (110) surface, which is known to exhibit high activity for CO2RR, also participates in the CO2RR to increase activity, and there was no degradation after CO2RR like Ag (111) surface.Although the lipid ligand can be removed by a strong base such as TMAH or a strong reducing agent such as NaBH4, 19 it does not seem to create a harsh environment, in which the ligand can be degraded, in the zero-gap CO2 electrolyzer using near the neutral electrolyte.
Comment 2: Collodial synthesis in OLA-type solutions require hazardous chemicals during the synthesis and the purification steps.On top of that, the yield of the synthesis are usually 30 % and below in terms of grams of metal used.After 50 hours, the selectivity and partial CO current density are not drastically different between the Ag-NPs and commercial Ag-PTFE samples.Could the authors provide further disucssion on the novelty in that aspect?
Response: Thank you for bringing this issue to our attention.The purpose of our study was to reveal the change in CO2RR property according to the increase in the hydrophobicity of the Ag catalyst by observing the behavior of electrolyte at three-phase boundary of the device in real-time.Although there are risks in colloidal synthesis of ligand exchanged metal nanoparticle and disadvantages of low yield.However, since structure and size is very homogeneous, ligand exchange Ag nanoparticles was used as a suitable model catalyst to achieve the purpose of this study.
The degradation rate for Ag-NP and Ag-PTFE catalysts after 50 hours of durability test at high cathodic overpotential was about 26 % and 36 %, respectively.Although the difference in performance reduction rate was not large, the current fluctuation tendency due to the behavior of electrolyte on the surface of catalyst was very large.In device operation, current fluctuation management is one of the important factors to improve stability of zero-gap device (reference: Journal of The Electrochemical Society, 2021, 168, 064507; Electrochemistry Communications, 2007, 9, 497-503).Related expression added in the revised manuscript.
Main manuscript Page 9, Line 3: Although the electrolyte discharged well, the current fluctuation was severe, and there was a slightly greater decrease in performance in relation to that of the Ag-NP catalyst.This is presumably due to the difference in the hydrophobicity homogeneity between the lipid ligand directly attached to Ag and PTFE randomly distributed around Ag. Voltage or current fluctuations in zero-gap electrochemical devices have a detrimental effect on long-term performance R9 degradation.Therefore, management of current fluctuation is one of important parts of electrochemical device research.
Comment 3: Refering the line in Page 5: " … To analyze the hydrophobic properties while maintaining the morphology of the catalyst, by leaving a part of the ligand, physical property analysis was performed using Ag-NP treated with TMAH for 2 h.…" a. Figure S2 shows the HR-TEM images of the "OLA-capped Ag-NPs" after 5 h of TMAH treatment to remove OLA, which results with the collapse of the conformal arrangement and the size of Ag-NPs.Hence, an alternative of 2 h TMAH treatment was suggested to preserve the NP stability.However, the evidence for the stability after 2h TMAH treatment is limitedso far, this evidence is limited to three individual Ag particles in Figure S3.Could the authors please provide further evidence with HR-TEM images of the selected process (2h-TMAH), comperable to the images in Figure S2 (or in Fig. 1a)? which are the TEM images used to build the histograms of 2h-TMAH sample?
Response: Thank you for your insightful comments.We added TEM images used to measure the particle size distribution of Ag-NP catalyst treated with 2 hours TMAH to the revised manuscript.It can be confirmed from the TEM images that the particle size or shape does not change even after 2 hours of TMAH treatment.Supplementary Figure 5. HR-TEM images of Ag-NP catalyst with 2 h of TMAH treatment for calculation of particle size distribution and average particle size.
Main manuscript Page 7, Line 7: Prior to electrochemical measurement, Ag-NP electrocatalysts was oxidized to obtain the oxide-derived electrode for removal of impurities on the surface of Ag-NP.Fourier transform infrared (FT-IR) spectroscopy was measured after oxidation to observe whether lipid ligand changes due to pre-oxidation process.For measurement under ideal conditions without influence of substrate (Supplementary Fig. 8: FT-IR spectrum of GDL), pre-oxidation process was performed in a half-cell system using a double polished silicon substrate.
Comment 7: Regarding Figure S8, a.The Faradaic efficiencies and partial current densities do not match.For ex. at 3.6 V, Ag black shows 70 % FE of H2 and 500 mA/cm 2 total cell current, which should be equal to 350 mA/cm 2 partial current for H2.Yet, Figure s8c shows only 275 mA/cm 2 .Could you please explain the inconsistency in the efficiency data?Please present a table of results for the tests conducted.b.There are not any error bars in any of the data.Is this a single test run?Is there any deviation amongst the GC injectionswhether there are multiple injections or not?And are there any duplicate or triplicate tests to minimize the deviation?
Response: We appreciate the reviewer's helpful comment.a. Several figures were miscalculated by confusing selectivity with faradaic efficiency.We added Figures with data errors corrected to the revised manuscript.In addition, a table summarizing the results of each CO2RR experiment was added to the supplementary information.b.We added an error bar to the CO2RR experiment in the revised manuscript.Error bars were obtained through triplicate tests.The corresponding contents was indicated in the experimental method section.In addition, the several values were corrected in the revised manuscript due to the additional experiments for adding error bars.Electrodes of the CO2 electrolyzer were prepared with 0.3 mg cm -2 of Ag catalysts on a 10-cm 2 gas diffusion layer (GDL) on the cathode side.(e) Schematics (low and high magnifications) of the triple phase boundary for the hydrophilic Ag black and hydrophobic Ag-NP catalysts in the CO2 electrolyzer.

R15
various particle size was confirmed.As shown in Supplementary Fig. 14, Ag-NP catalyst with average particle size of 7.33 nm exhibited the highest current density.This is consistent with the results of a previous study that Ag-NPs with a moderate size, between 5 and 10 nm, exhibit the highest CO2RR activity 30 .According to the result for CO2RR performance, Ag-NP catalyst with an average particle size of 7.33 nm was used for various analyzes to be discussed below.
Comment 10: In Figure S9, a.The partial current densities do not add up to the total value.For ex. at 3.4 V, the total j is around 300 mA/cm 2 , while the jCO is between 220 -250 mA/cm 2 1.
Comment 11: Regarding line " .. This is due to a decrease in the active surface area at the expense of suppressing the flooding phenomenon at a high cell voltage…" a. Are there any ECSA measurement to support this claim?b.For Ag-PTFE, the total cell current is less compared to Ag-NPs, which points to a higher resistance udner fixed voltage testing.Hence, the authors need to consider the electrical resistance contribution of the PTFE additive instead.

Response:
We appreciate your pointing it out.a.We measured CV for Ag catalysts in the non-Faradaic potential range.As shown in the Figure below, Ag black catalyst showed the largest active surface area compared to other catalysts, which was consistent with the total current density result of the CO2RR experiment.
assumption that the XANES signal of atoms collection is the linear sum of the XANES from individual components, which is valid under all conditions except the harsh conditions.In this sense, LCF is a useful approach to XANES analysis, and is generally very easy to perform.Sensitivity can be somewhat limited if done carefully, but can also be quite robust fitting method.
b. Metal reduction may occur during hard XAS measurement if the beam flux is large (reference: The Canadian Journal of Chemical Engineering, 2022, 100, 3-22; The Journal of Physical Chemistry C, 2021, 125, 11048−11057).Reduction can be conspicuous when exposed over tens of minutes.However, since XANES analysis was performed for a short time (~ 3 min), the effect of artifact due to beam damage was considered to be extremely small.Response: We are thankful to the reviewer for this valuable comment.Indeed, it was a typo; we corrected typo of Figure 4d in the revised manuscript.WCA after CO2RR at 3.4V of Ag black and Ag-NP catalysts was measured as 80.53° and 97.41°, respectively.The authors have thoroughly replied to the reviewer's questions, improving the manuscript.However, it needs to be clearer how the TEM analysis was done.First, the performance of the Agblack and Ag-NPs was tested in a zero-gap CO2 electrolyzer system, and the IL-TEM experiments were done using an aqueous electrolyte (CO2-saturated 0.1 M KHCO3).Even though the authors mentioned that the applied potential for those experiments was -1 V vs RHE, no information about the reached current density at that potential was included.Nevertheless, it is well-known that the current densities in aqueous electrolytes are limited due to the low solubility of CO2 in those electrolytes.The reaction environment is not the same in the zero-gap CO2 electrolyzer as when an aqueous electrolyte is used.Therefore, the observed changes in the Ag-black and Ag-NPs presented in Fig. 3a-d might not represent what happens in a zero-gap electrolyzer, especially when high current densities are reached (more than 300 mA/cm2).It will be helpful that the authors clearly mention why they analyze the catalyst changes in a system that does not represent entirely the same conditions as the zero-gap electrolyzer.
On the other hand, nothing is mentioned about the experimental part of how the Supplementary Figure 20-22 images were obtained.However, if those images were acquired after 50 h in the zero gap at 3.4 V, it would be better to include them in the main manuscript because they are more representative of what is happening to the catalyst morphology when they are tested in zero-gap electrolyzer.For example, suppose the images of the Figure 3d and Supplementary Figure 21 are compared.In that case, it is noticeable that the effects of cathodic corrosion are stronger (larger aggregates and more splitting of the Ag nanoparticles are observed) when the experiments are performed using a zero-gap electrolyzer (Supplementary Figure 21), where higher current densities are attained.Conversely, the changes after the CO2 electroreduction reaction in the aqueous electrolyte (Fig. 3c-d) are lesser, and this is most likely because of the low current density reached at -1 V vs RHE in the aqueous 0.1 M KHCO3.
The values presented in the Supplementary Table 1 do not match at all with the data presented in the Figure 2a, Supplementary Figure 13, and Supplementary Figure 15.The authors should check those values and mention to which Figures they are related.For example, with the first three catalysts (Ag black, Ag-NP, and Ag-PTFE-10wt%), the total current density values shown in the mentioned table are increasing, but in the last applied potential, there is a sudden decrease to ~30 mA cm-2, however, that is not observed in the Supplementary Figure 13a nor in the Supplementary Figure 15a, where the total current density is continuously increasing with an increasing cell potential.Moreover, the authors present in the Supplementary  2a and the Supplementary Figure 13d.Furthermore, it needs to be mentioned whether the column of partial current density values of the Supplementary Table 1 corresponds to CO or H2.
The Figure 2e represents a scheme of what is happening on the surface of the electrode during the CO2 electroreduction when a Ag-black and a Ag-NP catalysts are used.Still, no comments are mentioned about this figure in the main manuscript.
There is no caption for the Fig. 3g.
Include in the main text of the manuscript what the Supplementary Figure 7 represents.
Which experimental conditions were applied for the XRD characterization of the Ag-NP after the CO2RR in the Supplementary Figure 23?I recommend publishing the manuscript because it is of interest to the readership of Nature Communications only if the previous remarks are addressed in the main manuscript.
Reviewer #2 (Remarks to the Author): In overall, the article has improved after the 1st review, yet there are details in the data that needs to be revised and remarks to be addressed.Besides, the grammar needs to be improved to match the standards of the publisher.The authors are kindly requested to give answer to a few questions below: 1. What are the conditions of the "heat treatment", temperature, gas type, and heat/cool-rampdwell times employed in order to decompose the impurities remained after the ligand-exchange step?(following Comment 5 response) 2. What is the protocol for the "pre-oxidation step", could you please explain in detail?(following the Comment 6 response) 3. The authors need to be careful when they talk about the tests results of the "Ag-NPs before and after CO2RR" as the audience may think that these are the "Ag-GDE samples taken from the MEA after the CO2RR" -which is not the case... Proxy experiments are useful, and required for further analysis but still; this needs to be expressed clearly to the reader that they are not the exact samples.
a.The sample results for XRD, IL-TEM, EDS-Mapping, XANES, IR-Spectra, WCA are obtained on a different substrate in a different cell geometry -i.e.conventional batch-cells.Even though the applied potential may be the same (-1Vrhe), the current density hence the local-environment is totally different (jtotal < 30 mA/cm2 vs. >300 mA/cm2 for ex.).
4. Finally, the main message of the article needs to be clear in the abstract and title.If the flooding is investigated using in-situ techniques, then use of Ag particles with and without PTFE binder would suffice given the drastic change between the pristine vs. PTFE mixed catalyst layers.
Yet, the use of Ag-NPs (assisted with a ligand-exchange) is compared with Ag-PTFE mixture GDE.I still believe that, in this article, the novelty needs to be pronounced more effectively by postulating the phenomenon, which (to my understanding so far) is: • The distribution of PTFE around commercial Ag particles vs. local-coordination of OLA/TMAH ligands showed different hydrophobic behavior -locally.
• It is know that the local-reaction environment at higher current densities (> 200 mA/cm2) could expedite the diffusion-limited region to several hundreds of nanometers, demanding a greater mass transfer rate of CO2.Hence, we need to modify the surface within a few-hundred nanometer range to push the activity and preserve the stability.
• The three-phase-interface is preserved and pronounced better in the ligand-driven system.The interconnected ligand and nanoparticles can serve as the tunnels for the transfer of reactants and product, i.e.CO2 and CO, respectively.
• In comparison to the randomly distributed PTFE commonly used in literature, the results here are quite sufficient to elaborate the benefit of an advanced interface engineering to push the activity and preserve the stability.
• (Your " response c" for the Comment 12 is a good postulation, which could be inserted where you lay-out your discussion in the main article) Reviewer #3 (Remarks to the Author): Here, Ko et al. presented their work regarding improving the stability of Ag nanoparticles towards selective CO2RR with hydrophobic ligands.In general, I find the results from the author's experiments to be convincing of the mechanisms they had proposed.However, there are also several issues that I believe the authors need to address before the paper can be considered for publication, as I describe below.
The authors should provide low-magnification images of the nanoparticles after two hours of TMAH treatment in the supplementary information to support their claim of particle uniformity.
What is the difference between the two slices shown in Figure 1d?Are they from different locations in the tip?It is not clear to me what I am supposed to infer from the two slices.Why do the authors extract the profiles in 1(e) from an area which does not obviously show a silver nanoparticle?
The authors should provide scale bars from the slices.
Also related to the Figure 1(d).In line 134-135, the authors state that "Fig.1c-d show a needleshaped tip and its reconstructed 3D atom map containing several Ag-NPs and an iso-concentration surface of carbon." What does iso-concentration surface of carbon mean?
In several parts of the manuscript, the authors state that the ligands are attached to the edge of nanocrystals.
Line 128-129 Therefore, the CO2RR selectivity is expected to improve upon attachment of the ligand to the edge of the nanocrystal.
Line 174-175 The Ag-NP catalyst subjected to 2 h of TMAH treatment showed the highest performance because an appropriate amount of ligand attached on the surface, and there was no change in the icosahedron morphology while maintaining the active site of Ag.
Line 351-354 "Owing to the influence of the lipid ligands occupying the edge sites of the Ag NPs, cathodic corrosion and subsequent carbonate-ion adsorption at high cathodic overpotentials occurred to a comparatively lesser extent in the Ag-NP cathode than in the Ag black cathode." Here, I don't see any experimental evidence of edge attachment or references to previous work that show edge attachment.The authors should clarify.
Line 165-166 "Although the electrode is oxide-derived, the lipid ligand is well maintained." It is not clear to me what is expected or unexpected from the oxide-derived catalysts.Do the authors expect them to restructure?The authors should clarify.
Line 173-175 "The Ag-NP catalyst subjected to 2 h of TMAH treatment showed the highest performance because an appropriate amount of ligand attached on the surface, and there was no change in the icosahedron morphology while maintaining the active site of Ag." How do the authors know there is no change in the catalysis morphology during electrolysis at this point of the paper?Also, how do the authors know where is the active site of Ag?
Line 180 What is "Ag black"?I find no description of how this "Ag black" is prepared and its associated characterization.
Line 243-244 "The reaction was conducted at −1.0 V vs. RHE, near the highest partial current density." How did the authors determine that -1.0 V vs. RHE in the IL TEM cell replicates the behavior found in their electrolyzer?Here, the reaction environment and current densities are likely to be quite different between the two setups and may alter the morphological changes.The authors need to show that they can establish equivalence.Otherwise, they need to preface their discussion with a disclaimer that the two systems are not exactly equivalent.
Other issues that they need to clarify regarding the IL-TEM include whether they see bubble formation on the working electrode of the IL-TEM cell and if there is any impact of the TEM grid material.Au is also known to be selective towards CO.
Line 254-255 "This difference could be attributed to the rarely degradable lipid ligands that are attached to the edge of the nanoparticles." What does "rarely degradable" mean?Also, see above comment regarding edge attachment.
Minor Comments" Supplementary Figure 5.The authors state in the caption of Figure 5 that Supplementary Figure Image 5-5 is used in Figure 1(a).

R24
Reviewer #3 (Remarks to the Author): Here, Ko et al. presented their work regarding improving the stability of Ag nanoparticles towards selective CO2RR with hydrophobic ligands.In general, I find the results from the author's experiments to be convincing of the mechanisms they had proposed.However, there are also several issues that I believe the authors need to address before the paper can be considered for publication, as I describe below.

Comment 1:
The authors should provide low-magnification images of the nanoparticles after two hours of TMAH treatment in the supplementary information to support their claim of particle uniformity.
What is the difference between the two slices shown in Figure 1d?Are they from different locations in the tip?It is not clear to me what I am supposed to infer from the two slices.Why do the authors extract the profiles in 1(e) from an area which does not obviously show a silver nanoparticle?
Response: We appreciate the reviewer's helpful comment.We re-obtained TEM images of Ag-NP with TMAH treatment of 2h and 5h.For the existing TEM images, TEM analysis was performed after TMAH treatment of pristine Ag-NP coated TEM grid, but the newly measured TEM images were obtained by applying Ag-NP samples sonicated in a TMAH solution to TEM grids.Additionally, the particle size distribution for obtained TEM images for Ag-NP with 2h of TMAH treatment was recalculated.We added low-magnitude TEM images of TMAH-treated Ag-NP sample.
The two slices shown in Figure 1d are the same region of the APT tip.The left side is a slice marked with high contrast of Ag to show the location of Ag particles, and the right side is a slice marked with the same level of darkness throughout.Therefore, the profile in Figure 1e is the data obtained from the line profile in the area with the isolated Ag nanoparticle.Such information was added to the caption of Figure 1 in the revised manuscript.Please check the new Figure 1 and Supplementary Figure 1 on Page R3 and R8, respectively.

Comment 2:
The authors should provide scale bars from the slices.Also related to the Figure 1(d).In line 134-135, the authors state that "Fig.1c-d show a needle-shaped tip and its reconstructed 3D atom map containing several Ag-NPs and an iso-concentration surface of carbon."What does iso-concentration surface of carbon mean?
Response: Thank you for this helpful comment.We added scale bars to the slices in Figure 1c~d in the revised manuscript.The iso-concentration surfaces are commonly used to delineate phases in atomic probe datasets.These surfaces then provide spatial and compositional reference for proximity histograms, density and volume of atoms within a multiphase system.Please check the new Figure 1 on Page R3.

Comment 3:
In several parts of the manuscript, the authors state that the ligands are attached to the edge of nanocrystals.
Line 128-129 Therefore, the CO2RR selectivity is expected to improve upon attachment of the ligand to the edge of the nanocrystal.

Line 174-175
The Ag-NP catalyst subjected to 2 h of TMAH treatment showed the highest performance R25 because an appropriate amount of ligand attached on the surface, and there was no change in the icosahedron morphology while maintaining the active site of Ag.
Line 351-354 "Owing to the influence of the lipid ligands occupying the edge sites of the Ag NPs, cathodic corrosion and subsequent carbonate-ion adsorption at high cathodic overpotentials occurred to a comparatively lesser extent in the Ag-NP cathode than in the Ag black cathode." Here, I don't see any experimental evidence of edge attachment or references to previous work that show edge attachment.The authors should clarify.

Response:
We appreciate the reviewer's helpful comment.We used the corner site and edge site confusedly.The amidogen groups of oleylamine can ligand to the low-coordinated sites, especially corner sites, to minimize the surface energy.Thus, when the oleylamine ligands coordinate on the surface of the Ag-NPs, they are more likely to occupy the corner sites.We corrected expressions and added a reference to the binding of the ligand to the corner site of metal particles to the revised manuscript.
Reference: Chem.Commun., 2020, 56, 7021 Main manuscript Page 5, Line 24: To minimize the surface energy, the amidogen groups of the oleylamine ligands were attached to low-coordinated sites, especially the corner sites. 28When the ligands coordinated on the Ag-NP surface, they were more likely to occupy the corner sites.Therefore, the CO2RR selectivity was expected to improve upon attachment of the ligands to the corner sites of the nanocrystals.
Main manuscript Page 12, Line 13: This difference could be attributed to the remaining lipid ligands that were attached to the corner site of the nanoparticles.
Comment 4: Line 165-166 "Although the electrode is oxide-derived, the lipid ligand is well maintained."It is not clear to me what is expected or unexpected from the oxide-derived catalysts.Do the authors expect them to restructure?The authors should clarify.

Response:
We are thankful to the reviewer for this valuable comment.The expression in the Line 165-166 means that there was no structural degradation of the catalyst even though pre-oxidation treatment had been performed.We modified this expression more clearly in the revised manuscript.
Main manuscript Page 8, Line 13: Although the electrode was oxide-derived, the lipid ligand was not removed from the electrode surface.This indirectly suggests that there was little structural change in Ag-NPs.
Comment 5: Line 173-175 "The Ag-NP catalyst subjected to 2 h of TMAH treatment showed the highest performance because an appropriate amount of ligand attached on the surface, and there was no change in the icosahedron morphology while maintaining the active site of Ag."

R26
How do the authors know there is no change in the catalysis morphology during electrolysis at this point of the paper?Also, how do the authors know where is the active site of Ag?
Response: Thank you for your insightful comments.Through the IL-TEM experiment in Figure 3, it was observed that the morphology change of particles was very small even when potential was applied.In addition, relatively little particle agglomeration was observed even after durability experiments, at a high cell potential of 3.4 V.However, since the change in morphology had not been verified at the point of Figure 2, the expression seems to be erroneous.Therefore, we rephrased the sentences in the revised manuscript.
Main manuscript Page 8, Line 23: TMAH treatment for 2 h was predicted to be the optimal condition to maintain the icosahedral shape (Fig. 1d and Supplementary Fig. 5) while increasing the number of active sites of Ag through the appropriate amount of ligand removal.
Comment 6: Line 180 What is "Ag black"?I find no description of how this "Ag black" is prepared and its associated characterization.
Response: Thank you for bringing this issue to our attention.The black catalyst is a fine powder of metal.The name of black is due to its black color.
Comment 7: Line 243-244 "The reaction was conducted at −1.0 V vs. RHE, near the highest partial current density."How did the authors determine that -1.0 V vs. RHE in the IL TEM cell replicates the behavior found in their electrolyzer?Here, the reaction environment and current densities are likely to be quite different between the two setups and may alter the morphological changes.The authors need to show that they can establish equivalence.Otherwise, they need to preface their discussion with a disclaimer that the two systems are not exactly equivalent.
Other issues that they need to clarify regarding the IL-TEM include whether they see bubble formation on the working electrode of the IL-TEM cell and if there is any impact of the TEM grid material.Au is also known to be selective towards CO.

Response:
We appreciate the reviewer's helpful comment.When the potential applied to the cathode was measured in the zero-gap electrolyzer for the Ag black catalyst, about -1 V vs RHE (-1.610V vs Ag/AgCl) was applied at about 100 mA cm -2 , which was almost similar to the applied potential in the IL-TEM experiment.Therefore, the effect of the under potential can be observed to some extent through the IL-TEM experiment, despite the slightly difference reaction environments between the zero-gap electrolyzer and aqueous electrolyte.IL-TEM analysis is not affected by gas bubbles because it analyzes morphology changes before and after Rotation Disk electrode (RDE) experiments.And since the surface area of Au grid is very small compared to Ag black and Ag-NP catalysts, the amount of reaction is negligible at -1.0 V vs RHE.Such discussion and relevant experiment results were added to the revised manuscript.Main manuscript Page 11, Line 12: The reaction was conducted at −1.0 V vs. RHE (-1.610V vs. Ag/AgCl), near the partial current density of 100 mA cm -2 in a zero-gap electrolyzer experiment (Supplementary Table 1: the cathode potential in a zero-gap electrolyzer experiment with a Ag black catalyst obtained by adopting a reference electrode).Since many bubbles were generated at a high current density, morphology changes were observed after a long time (4 h) at a relatively low current density.Therefore, despite the slightly different reaction environments between the zero-gap electrolyzer and the aqueous electrolyte, the effect of the CO2RR potential on the morphology of the catalysts was observed to some extent through the IL-TEM experiment.

Reference
, a.In Fig 2b, the cell voltage shows a steady increase 80th hour from 3.1 V to ca. 3.5 V, and then experiences a sudden drop back to 3 V.Could the authors please explain why such a change has occurred?b.In Figure 2c-d & Fig S10, did the authors facilitated water removal externally by flushing with a liquid or inert gas media in any of the tests, at any time point?If yes, please specify the details in the experimental section.c.In comparison of the jCO performance of Ag-PTFE vs. OD-Ag-NPs treated with TMAH, the starting and ending partial CO current densities are identical (350 and 225 mA/cm2) along the 50 hours stability test.Yet, the water management property of the Ag-PTFE vs. OD-Ag-NPs treated with TMAH are different regarding the observed current spikes (150 vs 200 mA/cm2 min.point).The authors need to acknowledge the similarity in the performance (y-axis is over-strecthed in the Fig S10 in comparison to Fig. 2c! ) but they need to provide further evidence on, what sort of mechanism derives the difference in the local water management?(the last sentence of the paragraph in page 8 needs further evidence / discussion) 13.About the IL-TEM, a.The details of the sample preparation for the IL-TEM is not mentioned at any part of the manuscript or SI.Please clarify how are those steps following the disassembly of the MEA and preparing a TEM speciment for the post-mortem analysis?
3 e-f need to be reported in a table.

(Fig. 2 .
Fig. 2. Single-cell performance of the Ag-NP catalyst.(a) Selectivity of CO, and (b) CO partial current density versus applied cell voltage in a zero-gap CO2 electrolyzer for the Ag black and Ag-NP catalysts.(b, c) Durability test results of the Ag-NP catalyst in the zero-gap CO2 electrolyzer at (b) 150 mA cm -2 for 100 h and (c) 3.4 V for 50 h.(d) Durability test result of the Ag black catalyst in the zerogap CO2 electrolyzer at 3.4 V for 15 h.Selectivity of CO and H2 measured during the durability tests.Electrodes of the CO2 electrolyzer were prepared with 0.3 mg cm -2 of Ag catalysts on a 10-cm 2 gas diffusion layer (GDL) on the cathode side.(e) Schematics (low and high magnifications) of the triple phase boundary for the hydrophilic Ag black and hydrophobic Ag-NP catalysts in the CO2 electrolyzer.Main manuscript Page 16, Line 23: CO2RR was performed for 18 min at each cell voltage.

Fig. 3 .
Fig. 3. Morphology and phase change during the CO2RR.Identical location transmission electron microscopy (IL-TEM) images of the (a, b) Ag black and (c, d) Ag-NP catalysts (a, c) before and (b, d) after the CO2RR.(e, f) In-situ/operando X-ray adsorption near-edge structure (XANES) spectra at the Ag k-edge for the (e) Ag black and (f) Ag-NP catalysts during the CO2RR in the MEA-type electrolyzer, and its oxidation state distribution deconvoluted by linear combination fitting (LCF; magenta: metallic Ag, green: Ag + , and orange: Ag 2+ ).

Fig. 2 .
Fig. 2. Single-cell performance of the Ag-NP catalyst.(a) Selectivity of CO, and (b) CO partial current density versus applied cell voltage in a zero-gap CO2 electrolyzer for the Ag black and Ag-NP catalysts.(b, c) Durability test results of the Ag-NP catalyst in the zero-gap CO2 electrolyzer at (b) 150 mA cm -2 for 100 h and (c) 3.4 V for 50 h.(d) Durability test result of the Ag black catalyst in the zerogap CO2 electrolyzer at 3.4 V for 15 h.Selectivity of CO and H2 measured during the durability tests.Electrodes of the CO2 electrolyzer were prepared with 0.3 mg cm -2 of Ag catalysts on a 10-cm 2 gas diffusion layer (GDL) on the cathode side.(e) Schematics (low and high magnifications) of the triple phase boundary for the hydrophilic Ag black and hydrophobic Ag-NP catalysts in the CO2 electrolyzer.

Comment 16 :
WCA of 3.4 V is shown in Fig 4c but its value is missing in the chart of Fig 4d, instead 3.8 V was shown.Is this intentional or typo error?What is the contact angle of the electrode after 3.4 V?

Fig. 4 .
Fig. 4. Visual analyses of the influence of electrode hydrophobicity on the CO2RR.(a) A schematic representation of the zero-gap CO2 electrolyzer used for the in-situ/operando synchrotron computed tomography (CT) and cross-section tomography of the Ag-NP cathode.(b) Highmagnification synchrotron tomographs of the 3D structure of the Ag black and Ag-NP cathodes at various applied cell voltages (blue: electrolyte).(c) Water contact angle (WCA) images for the Ag black and Ag-NP cathodes at the initial state, 2.8 V, and 3.4 V. (d) WCA and water volume fraction versus the applied cell voltage in the zero-gap CO2 electrolyzer for the Ag black and Ag-NP catalysts.The water volume fraction in the electrodes at each applied cell voltage was estimated from the tomographs.

Table 1 .
and jH2 is almost zero, please clarify the breakdown of the calculation, preferably with table of results?Response: We appreciate reviewer's valuable comment.In several Figures, CO selectivity on the yaxis was incorrectly described as Faradaic efficiency.We added Figures with corrected y-axis captions to the revised manuscript.We also added a table of CO2RR results for all catalysts in the revised manuscript.Summary of experimental result for AEM zero-gap-type CO2 electrolyzer.

Table 1
Supplementary Table1of the Ag black, Ag-NP, and Ag-PTFE-10wt% always decrease at 3.4 V, but they increase at 3.6 V, which does not represent what is shown in the Figure a column with the CO F.E. (%) and CO selectivity (%) values.Can the authors explain the difference between those two values?According to my understanding, those values represent the same.Besides, the CO F.E. values in the